1. Field of the Invention
The present invention relates to an optical writing printer head for printing onto sheets of photosensitive print paper, which are intermittently fed at a constant pitch (paper feeding pitch) in a paper feeding direction, by applying light energy to (exposing) the print paper when the sheets of paper are scanned in a scanning direction intersecting the paper feeding direction, and to a printer using such a printer head.
2. Description of the Related Art
The phrase xe2x80x9cpaper feeding pitchxe2x80x9d as used herein indicates a sheet length of the print paper that is fed during a period starting from one exposure scanning operation to the subsequent exposure scanning operation while the sheets of print paper are intermittently fed in a paper feeding direction. The paper feeding pitch may be larger than a unit of length of the intermittent feed performed by a paper feeding means.
A conventional optical writing printer head of this type is provided with light source groups of three colors which are arranged in a scanning direction orthogonal to a paper feeding direction at a certain interval. The light source group of each color consists of a plurality of light sources practically having the same size which are arranged at the paper feeding pitch in the paper feeding direction. More specifically, in the conventional printer head of this type, a sub-region on a sheet of print paper is exposed to light from a first light source of one color. Then, light from a subsequent light source of the one color which is positioned at the downstream side of the paper feeding direction is further irradiated onto the sub-region while the sheet of paper is intermittently fed pitch by pitch. Thus, a print of a predetermined dot (a latent image formation) is performed. It should be noted that each light source is composed of a light source body formed of an LED (light emitting diode), and an aperture (diaphragm) formed before the light source body. The respective apertures of the light sources having substantially the same size are arranged at an equal interval along the paper feeding direction and the scanning direction, respectively.
However, in the conventional printer head of this type, as the pitch of the light sources coincides with a paper feeding pitch, the pitch of dots formed by the light sources coincides with the paper feeding pitch, so that the number of exposure controls for columns in the paper feeding direction becomes equal to or more than the number of rows in the paper feeding direction. If the print resolution (i.e., density of the dots to be printed on a sheet of print paper, or, in other words, the number of dots per unit of length in the paper feeding direction of the print paper) is high, the printing time will be hardly avoided from being long.
For example, if an exposure time per dot is 480 microseconds, the number of dots in the scanning direction is 320 dots per line, the inverse time in the scanning direction at the end of the scanning direction (the end of one row) is 0.05 seconds, and the number of dots (the number of rows) in the paper feeding direction is 144 dots, it will take the printing time of (480xc3x97320xc3x9710xe2x88x926+0.05)xc3x97144 seconds, i.e., approximately 29.3 seconds.
The present invention has been devised in view of the foregoing drawbacks, and an object of the present invention is to provide a printer head capable of reducing the printing time in the proportion of the print resolution (dot density), and an optical writing printer using such a printer head.
This object is attained with an optical writing printer head for printing onto sheets of photosensitive print paper, which are intermittently fed at a constant paper feeding pitch in a paper feeding direction, by irradiating light onto the print paper when the paper is scanned in a scanning direction intersecting the paper feeding direction. The printer head according to the present invention is provided with a first light source group in the paper feeding direction, the first light source group including N light sources of one color which are arrayed along the paper feeding direction at the standard pitch of 1/N of the paper feeding pitch, where N is a natural number which is 2 or larger.
In the printer head according to the present invention, N light sources in the first light source group are arrayed at the standard pitch of 1/N of the paper feeding pitch along the paper feeding direction. Therefore, a simultaneous exposure (print such as a latent image formation) with N dots can be carried out using the N light sources for each pitch of the paper feed.
That is, according to the present invention, when the printer head has a print resolution identical with that of the conventional printer head, the pitch of the N light sources will coincide with the pitch of light sources in the conventional printer head. On the other hand, while the conventional printer has a pitch of the light sources in coincidence with the paper feeder pitch, the printer head of the present invention has a pitch of the N light sources corresponding to 1/N of the paper feeding pitch. Thus, the paper feeding pitch amounts to N times the paper feeding pitch in the conventional printer, so that the print speed amounts to substantially N times that of the conventional printer. In other words, with the same print resolution as that of the conventional one, a higher print speed will be attained, and the printing time will be reduced in proportion to the print resolution (dot density).
If a printer having a paper feeding pitch identical with the paper feeding pitch of the conventional printer, in the printer head of the present invention, the pitch of the N light sources equals to 1/N of the paper feeding pitch,a mounting to 1/N as compared with the pitch of the light sources in the conventional printer head which coincides with the paper feeding pitch. Therefore, the printer head of the present invention can practically print at the print resolution increased by a factor of N (dot pitch of 1/N) during the same printing time as that of the conventional one. That is, with the same printing time as that of the conventional one, a higher print resolution can be achieved, and the printing time will be reduced in proportion to the print resolution (dot density).
The phrase xe2x80x9cstandard pitchxe2x80x9d as used herein means a minimum distance between the centers of the adjacent dots formed on the paper along the paper feeding direction (a dot pitch having the maximum resolution formed on the print paper). This coincides with a minimum distance between the centers of the adjacent light sources that form the adjacent light sources that form the adjacent dots. The xe2x80x9cscanning directionxe2x80x9d is typically orthogonal to the paper feeding direction, but it need not be orthogonal thereto in some cases so long as to intersect in direction. The number xe2x80x9cNxe2x80x9d is a natural number equal to 2 or 3, for example, but can also be 4 or more.
In terms of the N light sources, the xe2x80x9cone colorxe2x80x9d means a color corresponding to a specific wavelength area such as red, blue or green. Note that it includes a color consisting of light in a plurality of wavelength areas, also including white color. Typically, the N light sources have the ability of emitting beams having substantially the same sectional shape, sectional area (size), angle of divergence, maximum intensity, etc., but at least any of these factors may be different in some cases.
Preferably, the first light source group includes an upstream side light source of the one color at an interval corresponding to the standard pitch positioned at the upstream side of the N light sources with respect to the paper feeding direction, and the upstream side light source has a length in the paper feeding direction equal to (Nxe2x88x921) times the standard pitch. The phrase xe2x80x9cinterval corresponding to the standard pitchxe2x80x9d as used herein means an interval where the distance in the paper feeding direction between the center of the light source positioned at the most upstream side among the N light sources and the center of the most downstream side portion or region of the upstream side light source which is required to form on the print paper a dot having substantially the same shape as that of the dot formed by the most upstream side light source (the most downstream side light emitting region among the light emitting regions of the upstream side light sources, which has the same shape as that of the light emitting region of the most upstream side light source) coincides with the standard pitch. Typically, the upstream side light source having a length equal to (Nxe2x88x921) times the standard pitch and the N pieces of light sources at the downstream side thereof in the first light source group are arrayed in a line at the same position in the scanning direction, i.e., along the paper feeding direction, in a linear manner. In some cases, however, these light sources can be arranged in the position shifted in the scanning direction.
In this case, a xe2x80x9cpre-exposurexe2x80x9d is carried out on the print paper by the upstream side light source, and the paper can be pre-exposed by the N light sources positioned at the downstream side so that N pieces (N dots) of independent latent images may be formed thereon. The phrase xe2x80x9cpre-exposurexe2x80x9d as used herein means an irradiation of the light to photosensitive print paper on which a latent image can be formed when the light having the light amount equal to or more than a predetermined threshold value (threshold energy) reaches, and on which a latent image having the greatly varied density when it is developed relative to a change in the light amount can be formed at the level exceeding the threshold energy. The above light is irradiated having a level (light amount) less than the threshold at which a latent image can be formed on print paper but having a level near this threshold. Onto a portion of the print paper which is subjected to pre-exposure can form a latent image having the sensitively varied density when it is developed relative to a change in the light amount at the level exceeding the threshold value. Therefore, even if the beam intensity of the N pieces of light sources at the downstream side is relative low, the beam intensity of each of these N pieces of light sources is slightly changed and controlled, whereby a latent image having the desired density of N dots can be formed on the portion of the print paper which has been pre-exposed. This facilitates to photosensitize the print paper area that has been pre-exposed to the extent that it can be substantially saturated.
Preferably, the N pieces of light sources in the first light source group are composed of an aperture means having N pieces of apertures (diaphragms) arrayed at the standard pitch along the paper feeding direction, and N pieces of light source bodies arranged at an interval behind the aperture means so as to confront the apertures, respectively.
Further, preferably, the upstream side light source having a length increased by a factor of (Nxe2x88x921) is composed of: a light source body; and an aperture in an elongated hole form having a length equal to (Nxe2x88x921) times the standard pitch, the aperture being formed on the aperture means in position to confront the light source body.
As described above, the light source body is typically comprised of light emitting devices such as LEDs. Instead, other type of light emitting devices such as discharge tubes or semiconductor lasers may be available. The light source body is preferably like a surface illuminant in view of characteristics, but may be a point light source. A beam collimating means, a beam-condensing means, an image forming optical system or the like may be provided between the light source body and the aperture or the print paper, or a shutter capable of being electronically controlled such as a mechanical shutter or a liquid crystal shutter may be provided therebetween. To make up the upstream side light source having a length by a factor of (Nxe2x88x921), the light source body facing the elongated aperture may be formed of a plurality of (e.g., N pieces of) light emitting devices that are simultaneously controlled, or otherwise a single light emitting device as a whole.
Any color may be available for the color of the light source, including white color or colors of any color phase, as described above. However, as in the foregoing description, when the light source consists of a light source body and an aperture, the above-described structure capable of maximizing the dot resolution is preferably adopted for a light source having a color in a long wavelength region where the beam passing through the aperture can be easily diffused, i.e., red or red-like color,
One reason why the beam in a longer wavelength region can be more easily diffused than the beam in a shorter wavelength region is that the angle of divergence when a divergent beam (having a relatively large angle of beam divergence) having a beam diameter narrowed at the aperture (diaphragm) is refracted at a transparent protective layer on the surface of the photosensitive print paper less decreases with the longer wavelength of the light. In other words, this is because when the beam is refracted at the surface of the protective layer, and thereafter reaches a photosensitive layer (e.g., a layer to which photosensitive microcapsules are applied) positioned under the protective layer, the beam diameter will be increased with the longer wavelength of the light. In some cases, a diffraction that occurs at an outer edge of the aperture (opening) may be one reason thereof.
If light source of a second color is used as the light source, a second light source group different in color from the first light source group is juxtaposed with the first light source group. That is, the print head may be further provided with a second light source group having a different color. The second light source group is disposed in position shifted in the scanning direction relative to the first light source group, typically at either side in the scanning direction. The second light source group may be provided with light sources that are arrayed at the standard pitch. However, preferably, the second light source group includes a plurality of light sources that are arranged at a pitch corresponding to the paper feeding pitch in the paper feeding direction. Typically, the second light source group is arrayed in a line along the paper feeding direction. In some cases, however, it may be shifted in the scanning direction. In this case, some of the light sources belonging to the first light source group and some of the light sources belonging to the second light source group may be aligned in a line along the paper feeding direction. Typically, a light source of a color in the longer wavelength such as red is used as the first light source group in order to form a dot having a high resolution relative to the light beam in the longer wavelength where the dots tend to be expanded. Then, a light source of a color in the lower wavelength such as blue or green is used as the second light source group to provide the dot pitch having a low resolution in coincidence with the paper feeding pitch. However, in some cases, the reverse may be true. In the second light source group in which the light sources are arranged at the paper feeding pitch, each light source has a length in the paper feeding direction equal to (Nxe2x88x921) times the standard pitch so that the dots may be formed in the entire region in the paper feeding direction.
If a third color light source is used as the light source, a third light source group is typically provided at either side in the scanning direction over the entirety of the first and the second light source groups. In some cases, however, light sources in the third light source group may be arranged to be shifted along the scanning direction. In this case, at least some light sources out of the light sources in the third light source group may be arrayed in a line in the paper feeding direction together with at least some light sources out of the light sources in the first and the second light source groups. The third light source group (1) may include, as the first light source group does, N pieces of light sources at the standard pitch along the paper feeding direction, and the light sources may be composed of an aperture means having N pieces of apertures arranged at the standard pitch along the paper feeding direction, and N pieces of light source bodies (for example, LEDs) arranged at an interval behind the aperture means so as to confront the apertures, respectively; or (2) may include, as the second light source group does, a plurality of light sources arranged at the paper feeding pitch in the paper feeding direction.
In the former case, as is the case of the first light source group, preferably, the third light source group includes upstream side light sources of the further different color at an interval corresponding to the standard pitch positioned at an upstream side of the N pieces of apertures of the light source group, the upstream side light source having a length in the paper feeding direction equal to (Nxe2x88x921) times the standard pitch, and the upstream side light source is composed of: a light source body; and an aperture in an elongated hole form having a length equal to (Nxe2x88x921) times the standard pitch, the aperture being formed on the aperture means in position to confront the light source body. In this case, by way of example, when the first light source group consists of red light sources, typically, a blue light source is used as the second light source group, and a green light source is used as the third light source group. However, the reverse may be true with the colors of the second light source group and the third light source group. Further, the colors of the three light source groups may not be limited on the chromatic primary colors, but may be a combination of arbitrary colors. In some cases, a combination of colors may be further selected, considering the color-sensitivity of the print paper, characteristics available in colors of the light sources (intensity, beam diameter, or angle of divergence), etc.
In the latter case, preferably, each of the light sources in the third light source group has a length equal to (Nxe2x88x921) times the standard pitch in the paper feeding direction as is the case of the light sources in the second light source group. In this case, typically, when the first light source group consists of red light sources, blue light source and green light source are used as the second light source group and the third light source group, by way of example. Similarly to the above case, however, the color kinds and a combination thereof may be suitably selected. Further, depending upon the photosensitive characteristic of the print paper, a light source of an ultraviolet or infrared region which is substituted for the light source of a visible region may be used as a light source.
While the use of the light source group(s) of one kind to three kinds has been described, the number of light source group may be four kinds or more. Further, it is typical that the light sources in each light source group is preferably aligned in a line so that the most upstream side light source may be in the scanning direction. In some cases, however, some light sources may be in position shifted in the paper feeding direction in view of a whole length of a plurality of the light source groups arranged in the paper feeding direction, etc.
In order to achieve the above-described object, a printer of the present invention is equipped with a printer head having the foregoing structure. It is to be noted that the photosensitive print paper to be used for the printer is typically the one to which photosensitive microcapsules have been applied in view of easiness of handling and the like. The xe2x80x9cphotosensitive microcapsulexe2x80x9d used herein means a minute capsule so arranged as to be easily collapsed by pressure or hardly be collapsed (that is, a latent image is formed) upon reception of the light of a specific wavelength region, as well as a minute capsule in which a color-emitting material capable of color-emitting (that is, developing) a specific color upon the collapse is accommodated (typically, encapsulated). Note that the print paper can be of any other type which is photosensitive.